Since a series of aluminum phosphate molecular sieves and derivatives thereof were successively synthesized in 1982 by Union Carbide Corporation, US, in U.S. Pat. No. 4,310,440, aluminum phosphate molecular sieves and the heteroatom substituted derivatives are continuously one of research hotspots in the material field and catalyst field. This kind of SAPO molecular sieve synthesis technologies are characterized in that a silicon source, an aluminum source, a phosphorus source, and various template agents are employed in the synthesis, and the structural unit is composed of PO2+, AlO2−, and SiO2 tetrahedrons. Among this kind of molecular sieves, some molecular sieves having a microporous structure such as SAPO-34 have been successfully applied to the MTG, MTO processes and so on, and show excellent catalyzing performance.
The synthesis of SAPO molecular sieve generally employs a hydrothermal process, in which water is used as the continuous phase and the main solvent, and the molar ratio of water to organic amine template agent is generally larger than 10. The synthesis results in a large amount of waste liquids which are difficult to be utilized, increasing the loadings of environmental treatment. Meanwhile, the synthesis process has a relative low yield which is generally less than 80%. This is mainly due to the fact that the precursor formed by the synthesis raw materials has a relatively high solubility in the aqueous solution.
Taking SAPO-34 as an example, SAPO-34 is a chabasite-type (CHA) molecular sieve, having ellipsoidal cages of eight-membered rings formed by packing double six-membered rings in ABC manner and a three dimension crossing channel structure, wherein the pore size is 0.38×0.38 nm, and the cage size is 1.0×0.67 nm, belonging to microporous molecular sieve. The space symmetry group thereof is R3m, belonging to trigonal system. SAPO-34 is composed of four elements of Si, Al, P, and O, with a composition alterable in a certain range, generally n(Si)<n(P)<n(Al). The framework thereof is composed of SiO4, AlO4−, and PO4+ tetrahedrons, wherein three kinds of bonds of [Al—O—P], [Si—O—Al] and [Si—O—Si] are present, but no [Si—O—P] bonds exist.
Traditionally, SAPO-34 molecular sieve is generally produced by a hydrothermal synthesis process which uses water as the solvent and is conducted in a sealed autoclave. The components for the synthesis comprise an aluminum source, a silicon source, a phosphorus source, a template agent, and deionized water. The silicon source may be chosen from silica sol, active silica, and orthosilicate ester. The aluminum source may be active alumina, pseudo boehmite, or alkoxy aluminum. Preferable silicon source and aluminum source are silica sol and pseudo boehmite. Phosphorus source is generally 85% phosphoric acid. The template agent commonly used comprises tetraethyl ammonium hydroxide (TEAOH), morpholine (MOR), piperidine, isopropylamine (i-PrNH2), triethylamine (TEA), diethylamine (DEA), dipropylamine, and the like, and a mixture thereof.
In the traditional hydrothermal synthesis of SAPO-34, the molar amount of the organic amine template agent used is significantly less than the molar amount of water, and as the amount of the template agent gradually increases, both of the product yield and crystallinity decrease to some degrees, see Table 1 in Microporous and Mesoporous Materials, 2008, 114(1-3): 4163.
As another type of SAPO molecular sieve, RHO-SAPO molecular sieve having a RHO framework structure is formed by connecting α cages through double eight-membered rings, belonging to cubic crystal system, and the main channel is composed of double eight-membered rings, having an opening size of 0.36 nm×0.36 nm. In 1973, Robson, H. E. et al. firstly reported that a silicon-aluminum zeolite molecular sieve with a RHO structure was synthesized using Na+ and Cs+ as structure directing agents (Adv. Chem. Ser., 121, 106-115). In 1987, Rouse, R. C. et al. reported the discovery of one kind of natural ores having RHO structure (N. Jb. Miner. Mh., 1987, 433-440). Henceforth, BePO (Stud. Surf. Sci. Catal., 1989, 49, 411-420), AlGeO (Microporous Mesoporous Mat., 1999, 28, 139-154), BeAsO (1991, Nature, 349, 508-510), and GaSiO (J. Phys. Chem., 1995, 99, 9924-9932) molecular sieves having RHO structure were successively synthesized using Na+ and Cs+ as the structure directing agents. In 1998, Feng, P. Y. et al. reported that CoAPO—RHO, MgAPO—RHO, and MnAPO—RHO molecular sieves were synthesized using N,N′-diisopropyl-1,3-propanediamine as template agent (Microporous Mesoporous Mat., 23, 315-322).
The synthesis processes of RHO-SAPO molecular sieve mainly include a hydrothermal synthesis of RHO-SAPO with the participation of surfactant and a dry gel synthesis process without the participation of surfactant (see Chinese patent application No. 200910169329.X). For the hydrothermal synthesis process with the participation of surfactant, in one aspect, because the synthesis process employs water as the continuous phase and as the main solvent of the synthesis system, a large amount of waste liquid difficult to be utilized will be produced after the synthesis, increasing the loadings of environmental treatment; on the other hand, the synthesis process employs the relatively expensive surfactant, increasing the synthesis cost. In the dry gel synthesis process without the participation of surfactant, it requires to formulate a silicon-phosphorus-aluminum dry gel firstly, which involves a complicated process; the crystallinity of the RHO-SAPO molecular sieve obtained by this synthesis process is not high, and the obtained RHO-SAPO molecular sieve is generally difficult to be separated through the manner of washing and so on from the uncrystallized silicon-phosphorus-aluminum dry gel.
For solving the problems in the above described SAPO synthesis processes, the present inventors attempt to synthesize SAPO by a solvothermal synthesis process, that is, to synthesize SAPO molecular sieves by employing a non-water medium as the main solvent, and it is surprisingly found that, various kinds of SAPO molecular sieves can be successfully synthesized in the case where an organic amine is used as both the main solvent and the template agent of the synthesis system, in the presence of only a small amount of water.